Neutral point connected apparatus providing compensation to an AC power line

Abstract

Apparatus for injection voltage compensation in series into an electric power line includes an inverter or other controlled voltage source which wye connects the secondary windings of a three-phase transformer to the load end of the electric power line. The transformer, which could already be present for meeting load requirements, has its primary winding connected to the source end of the electric power line and sustains the saturation currents rather than the inverter in response to faults on the source side of the transformer. Preferably, a three-phase inverter with a common dc link is used. For active power line conditioner (APLC) action, a second, also preferably a three-phase, inverter is connected in parallel to the load end of the electric power line and to the common dc link. This parallel inverter provides harmonic and reactive power compensation and also supplies the real power requirements of the wye connection inverter.

What is claimed is: 
     
       1. Apparatus for providing compensation in a three-phase electric power line between a source end and a load end, said apparatus comprising: three-phase transformer means connected in said three-phase power line between said source end and said load end and having three primary windings connected to said source end of said three-phase electric power line and having three secondary phase windings; and   inverter means wye connected in series with said three secondary phase windings of said transformer means to said load end of said electric power line.   
     
     
       2. The apparatus of claim 1 wherein said three secondary phase windings of said transformer means have first ends connected to a corresponding phase of said load end of said electric power line, and said inverter means comprises a three-phase inverter having ac terminals connected to second ends of corresponding ones of said secondary phase windings of said transformer means and having dc terminals and a common dc link with common dc energy means connected to said dc terminals. 
     
     
       3. The apparatus of claim 2 wherein said dc link includes a dc energy device which exchanges power with the electric power line. 
     
     
       4. The apparatus of claim 2 adapted for use with an electric power line in which said load end includes a neutral conductor, and wherein said inverter means includes a neutral inverter pole having an ac terminal connected to said neutral conductor and dc terminals connected to said common dc link. 
     
     
       5. The apparatus of claim 4 wherein said inverter means includes filter means. 
     
     
       6. The apparatus of claim 5 wherein said filter means includes inductors connected in series with said ac terminals of said inverters and shunt capacitors connected to a common node. 
     
     
       7. The apparatus of claim 2 wherein said inverter means includes filter means. 
     
     
       8. The apparatus of claim 7 wherein said filter means comprises inductors connected in series with said ac terminals of said inverter and shunt capacitors connected to a common node. 
     
     
       9. The apparatus of claim 1 wherein said three secondary phase windings of said transformer means have first ends connected to said load end of said electric power line and wherein said inverter means comprises three single-phase inverters each having a first ac terminal connected to a second end of a corresponding secondary phase winding of said transformer means, second ac terminals connected to the second ac terminals of the other single-phase inverters, dc terminals and dc energy means connected to said dc terminals. 
     
     
       10. The apparatus of claim 9 adapted for use with a power line in which said load end has a neutral conductor, and wherein said neutral conductor is connected to said second ac terminals of said three single-phase inverters. 
     
     
       11. The apparatus of claim 9 wherein said inverter means includes filter means. 
     
     
       12. The apparatus of claim 11 wherein said filter means includes inductors in series with said first and second ac terminals of said single-phase inverters and shunt connected capacitors connected to a common node. 
     
     
       13. The apparatus of claim 1 wherein said secondary phase windings of said transformer means have first ends connected to a corresponding phase of said load end of said electric power line and said inverter means comprises first inverter means connected to second ends of said secondary phase windings of said transformer means forming a neutral point for said wye, and second inverter means connected between said neutral point and each phase of said electric power line between said first ends of said secondary phase windings of said transformer means and said load end of said electric power line. 
     
     
       14. The apparatus of claim 13 wherein said first inverter means is a first three-phase inverter having ac terminals connected to the second ends of corresponding ones of said secondary phase windings of said transformer means and having a common dc link with common dc energy means, and wherein said second inverter means comprises a second three-phase inverter having ac terminals connected to corresponding phases of the load end of said electric power line and having dc terminals connected to said common dc link. 
     
     
       15. The apparatus of claim 13 wherein at least one of said first and second inverter means comprises a three-phase inverter with a common dc link having a common dc energy device. 
     
     
       16. The apparatus of claim 13 wherein said inverter means includes filter means. 
     
     
       17. The apparatus of claim 16 wherein said filter means comprise LC filters connected to ac terminals of said inverter means. 
     
     
       18. The apparatus of claim 16 adapted for use where said load end of said electric power line includes a neutral conductor and wherein means connects said neutral conductor to said neutral point of said wye. 
     
     
       19. The apparatus of claim 1 adapted for use with an electric power line having a neutral conductor at said load end and wherein said inverter means includes means connecting said neutral conductor to said wye.